The heating system in a building can have a significant impact on its energy use. Most buildings use electricity for lighting, computers, and other equipment, but heating systems can use a variety of fuel sources.
The table below shows six different scenarios for heating systems. For each building, the operation and thermal envelope are the same, so the heat load for each building is identical. The differences among the buildings are solely in the type of heating fuel and the equipment used for heating. As you can see, buildings A, C, and E are all in the same ballpark for Source Energy (1313, 1264, 1256), meaning these three heating system/ fuel type combinations have about the same efficiency. But, Buildings B and F have inefficient heating systems, and this shows in their Source Energy (1909, 3140).
When you compare different buildings especially, with different fuel types and different heating systems, Site Energy is not a fair comparison. Electricity Site Energy will always look better than Natural Gas Site Energy, because electricity is a secondary energy and natural gas is a primary energy. By using Source Energy, we capture the total primary energy (or, raw fuel) that is required by each building, providing a more complete picture of its energy needs.
In summary, if you only looked at Site Energy, Building F would be superior to Building A, but by looking at Source Energy, you can see Building F is much less efficient.